Candida albicans is a widespread human fungal pathogen that causes high rates of mortality during systemic infections, and is particularly dangerous for immunocompromised AIDS patients. Because fungi such as C. albicans are eukaryotes, development of antifungal therapeutics that is non-toxic to humans is often challenging. Recently, RU109 was identified as the enzyme that catalyzes acetylation of histone H3 lysine 56 in the budding yeast, Saccharomyces cerevisiae. Mutants lacking Rtt109 are viable, but is slow growing and extremely sensitive to genotoxic agents. Notably, no close homologs of RTT109 genes are found outside of fungal species, and Rtt109 proteins do not contain signature residues found in the other families of histone acetyltransferase (HAT) enzymes. Therefore, we hypothesize that we can identify small molecules that inhibit Rtt109 function without substantial effect on other HAT enzymes. Furthermore, as Rtt109 homologs are restricted to fungi, they represent promising targets for small molecule therapeutic intervention with minimal toxicity for mammalian hosts. In this revised proposal, I aim to elucidate the role of Rtt109 in pathogenesis by C. albicans and to discover Rtt109 inhibitory compounds that are efficient in vivo. I have confirmed the functional conservation of the C. albicans Rtt109 enzyme, because it is essential for H3K56 acetylation and for resistance to genotoxic agents. I will test whether C. albicans rtt109-/- mutants display increased sensitivity to macrophages in vitro and whether they are pathogenic in the established murine candidiasis model. Second, I will screen a library of small molecules for inhibition of histone acetylation by Rtt109 in vitro. To do this, we have developed a high-throughput assay which will allow quantitative assessment of histone acetylation by purified, recombinant Rtt109, detected with an anti-H3K56-acetyl antibody. Finally, I will begin to characterize candidate compounds that are non-toxic to mammalian cells for their effects on histone modification in Candida, and on pathogenesis in mice. PUBLIC HEALTH RELEVANCE: Candida albicans is a pathogenic fungus that is particularly dangerous to immunocompromised individuals, including AIDS patients. Recently, a new enzyme was discovered that is important for normal growth of fungi. I propose to study how this enzyme contributes to growth and virulence of Candida albicans. I will also identify compounds that inhibit this enzyme, with the goal of developing new therapeutic approaches to combat fungal infections.